Summary The Air Canada Regional Airlines DHC-8-100 aircraft with 35passengers and a crew of three was on a flight from Sydney, Nova Scotia to Halifax, Nova Scotia. The flight took off at 1547 Atlantic daylight time, and during the climb out at about 6000feet, the first officer observed ice in the air inlet duct of the right engine (PrattWhitney120A). About five seconds later the right engine flamed out. The aircraft was being operated with the engine ignition on and the engine recovered almost immediately. Approximately two minutes after the engine had recovered, the right engine flamed out and recovered again. After completing the required checklist procedures, the first officer checked the air inlet duct and observed that the ice had disappeared. The captain then checked the left engine and saw that ice was also present in the left air inlet duct. A few minutes later, as the aircraft was reaching its cruise altitude at 14000feet, the left engine went through a similar flame-out and recovery sequence. The aircraft continued to its destination without further incident. Ce rapport est galement disponible en franais. Other Factual Information The aircraft had arrived in Sydney the previous evening and was scheduled to depart the next morning. On approach at Sydney there were traces of light rime icing with no appreciable accumulation, and aircraft anti-ice and de-ice systems were used. The wing and tail leading edges were clean on arrival at the ramp. The Sydney weather was blowing snow with a temperature of minus 1C and a dew point of minus 4C. A short time after arrival in Sydney, the aircraft was placed in an unheated hangar, where the temperature was slightly above freezing. Due to a snow storm in the area, all morning flights out of Sydney were canceled. The occurrence aircraft was removed from the hangar and positioned on the ramp, into the wind, at 1450 Atlantic daylight time (ADT)1 on the day of the occurrence. The weather was reported to be as follows: wind 340 true at22, gusting to31, knots, visibility 1mile in light snow and blowing snow, temperature -1C, and dew point -5C. The first officer completed the pre-flight inspection at 1500. This included a tactile inspection of the engine plenum areas through each by-pass door; no contamination was found. A visual inspection of the engine air inlet ducts was conducted from the ground. However, the lower portion of the inlet ducts would not be visible from this vantage point. Immediately after the pre-flight inspection, the ground handler installed the engine air intake plugs to prevent snow from accumulating inside the engine air inlets. He gained access to the air inlets by standing on the baggage cart and examined the air inlet ducts visually and by running his hand inside them before installing the plugs and again after they were removed (at1520) for the start of the right engine to warm the aircraft cabin prior to passenger boarding. The ground handler did not see or feel any contamination during the installation and removal of the inlet plugs. The flight crew carried out visual inspections of the engine air inlets from the cockpit during the de-icing operation, prior to engine start, and just prior to the start of the take-off roll and no contamination was observed. The Bombardier Aerospace Engine Lower Cowl Intake Icing Inspection Training Guide outlines the precautions and inspections recommended to be carried out on the aircraft during overnight parking, prior to departure and take-off during inclement weather. At the time of the occurrence, the guide contained the following recommendation on engine inlet inspection: Check inside the engine inlet with a flashlight for any signs of ice, snow or slush. This should include visual and tactile inspections of all areas below the intake heater adapter, plenum chamber and diffuser areas. Any ice, snow or slush must be removed with a snow-brush or similar tool. If necessary pre-heat lower cowl to remove ice. The training guide did not contain a specific recommendation to perform a tactile inspection of the inlet duct area. After the passengers were boarded, the aircraft was de-iced, and, at 1536, the engines were started for departure. The engine ignition was selected ON after engine start in accordance with the DHC-8-100 Aircraft Flight Manual (AFM), which requires that engine continuous ignition must be turned to the ON position when the aircraft is operating in icing conditions. The engine anti-ice systems were also selected ON after engine start and operated continuously throughout the flight. After engine start and while taxiing to position for take-off on Runway01, the crew turned the heat up in the cabin, which resulted in a lot of condensation and vapor in the cabin. The lavatory smoke alarm then activated, and, while this problem was being resolved, the aircraft continued to taxi for position for the take-off. After arriving in position for take-off at the end of the runway, the crew stopped the aircraft momentarily until the problem was rectified. The airport ramp condition at the time of departure was reported as 95percent bare and wet, 5percent bare and dry. The aircraft began its take-off roll at 1547, eleven minutes after the engines were started. Airframe de-icing systems were not used during the departure and climb to cruise altitude because no airframe icing was observed by the crew. Propellor heat was turned on during the climb-out. When the right engine flamed out the first time, the right engine control unit (ECU) reverted to MANUAL. After the engine recovered, the crew re-selected the ECU to NORMAL and about 30seconds later, the right engine flamed out again with the ECU again reverting to MANUAL. Following the second flame-out of the right engine, the crew did a complete review of their options, including returning to Sydney; however, due to the weather conditions in Sydney and the published AFM limitations with one ECU inoperative, the crew elected to continue to Halifax. The crew then consulted with company maintenance personnel who suggested that the ECU circuit breaker be cycled. The crew elected to wait until after top of climb to do this. However, just as they were reaching their cruise altitude, the left engine went through a flame-out and restart sequence with the left engine ECU reverting to MANUAL. With both ECUs now in MANUAL, the crew cycled the ECU circuit breakers and re-selected the NORMAL position one engine at a time. With the aircraft clear of cloud, operating normally, and with no further sign of ice in the inlet ducts, the crew did not deem it necessary to declare an emergency. The crew described the ice that they observed in the engine air inlet ducts before the engines flamed out. The ice started three to four inches aft of the intake lip, it spread aft along the bottom and side as far aft as they were able to see, and it was very white, with a consistency similar to that of rime ice. A special weather observation taken three minutes after the take-off was as follows: wind 340true at24, gusting29, knots, visibility 1mile in light snow and blowing snow, overcast ceiling at 700feet, temperature -1C, and dew point -5C. The temperatures remained the same throughout the afternoon. The upper wind charts predicted the following temperatures, in degrees Celsius: 6000feet,-7; 9000feet,-7; and 12000feet,-12. The significant weather charts valid for the time of departure indicated that there would be moderate mixed icing from the surface to 11000feet for the Sydney area. As a result of the delay in the reporting of this incident to the TSB, the flight data recording associated with the occurrence flight was overwritten and was not available for investigation purposes. There have been other DHC-8 in-flight engine flame-outs attributed to ice ingestion and/or airflow disruption due to the dislodging of an ice sheet. Two of these flame-outs (TSB/CASB reports A87O4219 and A88A0039) have been attributed to ice accumulation during ground operations. There are indications that another flame-out event, TSB Report A99A0160, could have been related to in-flight ice accumulation in the aft plenum of the engine air inlets; however, the incident was not investigated thoroughly. In an attempt to prevent further occurrences, modified procedures were developed for the 2000/2001 fall/winter/spring seasons to ensure that the engine air inlets were free of contamination on the ground. These procedures were distributed to operators by Bombardier Aerospace through a service letter dated 20September2000 and a revised CDROM training guide. Highlights of the information are as follows: Check inside the engine inlet with a flashlight for any signs of ice, snow or slush. Check for ice accumulation inside the nose cowl through engine bypass doors. If frozen contaminant is discovered, ensure that the lower cowl intake is clear prior to engine start. Check inside the engine inlet with a flashlight for any signs of ice, snow or slush. Check for ice accumulation inside the nose cowl through engine bypass doors. If frozen contaminant is discovered, ensure that the lower cowl intake is clear prior to engine start. Air Canada Regional Airlines (ACR) adopted these modified procedures and incorporated them into their flight crew and ground handler Standard Operating Procedures (SOPs) and training programs. Both flight crew members and the ground handler were trained in these procedures prior to the occurrence. Subsequent to this occurrence, personnel from Bombardier Aerospace, ACR, Pratt Whitney Canada, Transport Canada (TC), and the TSB undertook a systematic fault analysis process called Relentless Root Cause Analysis (RRCA) to establish why the engines had flamed out. This process involved identifying the factors that may have contributed to the event and then analyzing each factor to determine what, if any, role it played. One of the factors that was identified as being a possible contributing factor was that three of the four drain holes (two in each inlet duct area just forward of the skijump) were completely blocked with green paint and the fourth was partially blocked with paint. It is probable that the holes were painted over during cowling refurbishment by the operator. During the process it was agreed that the engine flame-outs were caused by ice in the engine air inlet ducts lifting up as a solid sheet, interrupting the airflow to the engines and causing them to flame-out. As a result of the RRCA, two possible scenarios were postulated which could have created the conditions which led to the engine flame-outs. The first scenario was that there was ice in the inlet ducts prior to engine start and the second scenario was that ice accumulated in the inlet ducts after engine start. In the first scenario, it was hypothesized that ice/snow, which had accumulated in the intakes on the previous flight, melted while the aircraft was in the hangar overnight and then pooled due to blocked drain holes. This residual water went undetected by the flight and ground crews and froze as a sheet of ice on the bottom of each intake after the aircraft was removed from the hangar. After the aircraft was airborne, these sheets of ice broke free and lifted up as a solid sheet, momentarily interrupting the airflow to the engines and causing them to flame-out. A National Transportation Safety Board (NTSB) report on a Grumman G-159 (G-1) accident that occurred on 19July2000, in which engine icing was discussed, contained the following excerpt from the G-1FlightManual, AppendixA, Adverse Weather/Abnormal Atmospheric Conditions section, which stated in part: Engine/propeller icing can occur without wing icing. A turbine engine operating in an air mass with an ambient temperature below 8degreesC may experience engine icing; this is caused by the temperature drop associated with the reduction in pressure between that of the air mass and the pressure at the propeller disk and/or first stages of the compressor. As air is drawn past the propeller or into the engine, moisture condenses into droplets. Theses droplets, due to their inertia, cannot follow the airflow around the propeller, guide vanes, or compressor blades. Instead, they strike the metal parts and freeze The excerpt from the G-1 Flight Manual raised the question as to whether the same thing could have occurred in the DHC-8-100 inlet. In response to this question, Bombardier Aerospace carried out a Computational Fluid Dynamic (CFD) analysis (second scenario). Two different type nacelle inlet installations were analyzed and compared, the pitot nacelle inlet (DHC-8-100) and an annular nacelle inlet (similar to the type found on the G-1) using ISA standard atmosphere conditions at 9000feet altitude and a temperature of -3C. The findings of this comparative analysis were as follows: The temperature rise across the propeller disk is small, approximately 1C, for both applications. The temperature increase due to flow deceleration between the propeller and the inlet is significantly larger than that imparted by the propeller, approximately 4C, compared to the annular inlet where there is a slight decrease in temperature. There is a slight drop in temperature as the air enters the DHC-8-100 inlet and then the temperature increases a slight amount over the length of the inlet before entering the engine compressor. In the annular inlet, there is a sharp decrease in temperature of approximately 10C as the air enters the inlet and then the temperature climbs approximately 7C over the length of the inlet before entering the engine compressor. As part of the CFD analysis, Bombardier Aerospace also looked at the impact on snow/ice development in the DHC-8-100 inlet by calculating the length of time required for snow in the inlet flowfield to melt, the length of time a particle takes to travel from the inlet to the compressor face, and determining what effect the temperature/dew point spread on 03April2001 may have had. The findings from this analysis were as follows: Time to melt ice crystal/snowflake is approximately 12seconds. The residency time of a particle in the inlet is 20milliseconds. The air temperature and the dew point on 03April2001 were -1C and-5C, respectively. Because the temperature of the air increases as it enters the inlet, the temperature would have been about 6degrees Celsius higher than the dew point. The conclusions from the above analysis were that snow/ice would not have sufficient time to melt before being ingested in the engine compressor and due to the spread between the temperature and dew point in the inlet, any water vapour in the airstream would not have precipitated out in the form of fog or snow. In another CFD analysis, Bombardier Aerospace conducted a particle trajectory analysis for the DHC-8-100 inlet for in-flight operations, on the ground with engines at idle power, and on the ground with engines stopped. The conclusions from this analysis were as follows: Significant impingement of ice particles can occur on the bottom of the inlet duct on the ground when the aircraft is pointed into the wind during a snow storm with the engines off. There is no impingement of snow/ice particles on the bottom of the inlet duct on the ground when the aircraft is pointed into the wind during a snow storm with the engines at idle power. There is no impingement of snow/ice on the bottom of the inlet duct in flight when the duct is free of ground ice. Impingement of snow/ice particles can occur on the bottom of the inlet duct in-flight when a forward facing step is created by the breaking of a pre-existing sheet of ice. Some ice accretion can occur on the forward facing step of a pre-existing sheet of ice. Based on the CFD analysis, Bombardier Aerospace offered the following to explain the observation of an ice build-up in the inlet duct during the flight: The ice sheet may have broken just aft of the de-icing boot because of activation of the de-icing boot or because of high normal and shearing stresses in the thin edge (wedge) of the ice sheet. After the ice sheet broke away there would be a forward facing step which could cause a localized change in air pressure and flow angle, resulting in a small amount of observable ice accretion which would not affect engine performance. Applying the lumped-capacity heat transfer analysis and forced convection principles, Bombardier Aerospace computed that the time required for water on the bottom of the inlet duct to freeze is approximately 30minutes under the conditions of 03April2001. The assumptions used for this computation were: drain holes blocked, water had a depth of 0.6inch (about half of the maximum possible depth), water was assumed to be at 1C, outside air temperature at -1C, and air blowing in inlet at 10knots due to the wind. During the winter of 2001/02 ACR reported 12incidents in which ice accumulated in the engine inlets during flight. One of these incidents was on 05December2001. In this incident, the aircraft had been parked on the ramp overnight in Charlottetown, Prince Edward Island, with the engine air inlets protected. The crew completed a pre-flight inspection of the aircraft and an engine intake inspection in accordance with the latest procedures. The engine air inlets and airframe were reported to be dry and free of any form of contamination. The 0600 Charlottetown weather was as follows: wind 120 at 5knots, visibility 15statute miles, temperature minus 2C, dew point minus 5C, ceiling 3000feet scattered and 7000feet overcast. The aircraft departed at 0628 and at 3100feet entered cloud and encountered light snow. At 11700feet, light rime ice was observed and as the aircraft leveled at 12000feet, both the airframe and engine de-icing boots were selected on (ice was observed in the engine air inlet ducts at this time). The boots were selected on again during cruise and just prior to the start of descent into Halifax; the de-icing boots were clean at the end of each cycle. The aircraft landed in moderate snow at 0655. Because of the icing conditions encountered, the engine air inlets were inspected 10minutes after landing, and inch of ice was found on the 'skiramp' areas of the air inlet ducts, just aft of the drain holes. One-half inch of ice was also observed on the radome. The 0700 Halifax weather was as follows: wind 130 at 6knots, visibility 1statute miles in snow showers, temperature and dew point minus 1C, and ceiling 2800feet overcast. On 03January2002, the airline sent a letter of concern to the manufacturer regarding the icing noted in the intakes following the 5December2001 flight. In a response to the airline, on 28January2002, Bombardier stated that it would not anticipate that the icing noted would cause any operating anomalies. It was determined that during initial aircraft certification, the DHC-8-100 met or exceeded the certification criteria for intake induction icing. These criteria state that, . . . each engine, with all icing protection systems operating, must operate throughout its flight power range (including idling) without the accumulation of ice on the engine components that adversely affects engine operation or that causes a serious loss of power or thrust in continuous maximum and intermittent maximum icing conditions . . . On the 03April2001 flight, the aircraft was operating well within (below) the maximum icing conditions threshold when the engines flamed out. Since the 1999 occurrence, Transport Canada Continuing Airworthiness has been actively monitoring the DHC-8-100 engine air inlet icing issue. Following the 03April2001 occurrence, TC has closely followed the RRCA process, operator actions, and the response and actions of the aircraft manufacturer. TC has also conducted a formal risk assessment considering at least three scenarios. TC concluded that the probability of a double engine flame-out and failure to restart at least one engine due to in-flight ice contamination to occur was improbable/unlikely, because over the total flying hours of the entire global DHC-8-100 fleet (more than 10million flight hours) there has been no such occurrence. By combining the hazard severity and the hazard probability TC's Risk Assessment determined the risk to be low. Risk determination would usually require that the duration of exposure to a hazard be factored with the severity of the event under consideration and with the probability of that event. In this type of occurrence, the exposure to the hazard (in-flight ice contamination in the air inlet ducts) can only occur when icing conditions exist and an aircraft is flying through or in such conditions. Therefore, determining the duration of exposure to conditions that would result in ice accumulation and possible engine failure is virtually impossible.